In the past, the maximum and minimum pressures of a compressed air generating system including an air compressor and storage reservoir were normally adjusted by means of control devices which were arranged between the air compressor and the storage reservoir. Previously, pressure control devices were employed to interrupt any further pressure build-up in the storage reservoir when the adjusted maximum pressure is reached. One manner of accomplishing this was to have a valve divert the flow of the compressed air generated by the air compressor into the atmosphere. Another way was to switch the air compressor to its idling position. In the latter case, switching to the idling phase was accomplished by establishing a fluid connection between the suction chamber and the compression chamber of the air compressor.
Such pressure regulating systems are well known and are fully described in German Pat. No. 1,550,138 and allowed German application No. 27 26 494.
However, when discharging into the atmosphere, the pressure control valve must be installed in the unit in such a way that a length of spiral tubing should be connected to the discharge end of the air compressor. In practice, a length of 1.5 to 2 meters of tubing will sufficiently cool the heated compressed air so that the temperature at the intake nozzle of the pressure regulating valve will not exceed +150.degree. C. This ensures that the entire capacity of the air compressor will be completely evacuated into the atmosphere during each discharge cycle, and likewise that the tubing will be filled to maximum by atmospheric pressure on each intake stroke.
However, there is a disadvantage in switching the air compressor to its idling position in that the air compressor is only partly cooled. It will be appreciated that during the idling operation there is little, if any, air being sucked into the compressor so that part of the cooling air need for cooling the air compressor is therefore lost. This is also the case in self-stabilizing types of air compressors.